The present invention relates generally to stainless steel orthopedic implant devices and, more particularly, to a surface treatment process applicable to such devices, wherein both the fatigue and corrosion properties of an orthopedic implant device are enhanced.
More specifically, the present invention relates to orthopedic implant devices commonly used by orthopedic surgeons to repair and replace fractured and deteriorating bones and joints. For example, U.S. Pat. No. 4,612,920, issued to Lower, discloses a compression hip screw of the general type to which the surface treatment process of the present invention is applicable. Other orthopedic devices that the present invention would be applicable to include, but are not limited to, hip prostheses, bone plates, intramedullary nails, and other fracture fixation devices fabricated from stainless steel.
Representative of the function of many orthopedic implant devices, the aforementioned compression hip screw rigidly connects a femoral head to the remaining portion of the femur despite a fracture in the area of the femur neck. During a normal walking cycle, substantial loads are imparted to and transferred by the compression hip screw. Consequently, it is desirable to reduce the possibility of a component failure that might require the patient to undergo further surgery.
A variety of stainless steel alloys are used for fracture fixation devices and are chosen on the basis of their high strength, ductility, fracture toughness, biocompatibility, and corrosion resistance. It is generally known that the fatigue and corrosion properties of orthopedic implant devices fabricated from these alloys can be affected by different surface treatment processes. For instance, one such process includes the steps of mechanical grinding, sisal buffing or color buffing, electropolishing, and passivation.
Recently, orthopedic implant devices have been surface treated by shot blasting with alumina, i.e., a form of aluminum oxide having a hard crystalline structure. Alumina is typically used as an abrasive and includes a sharp, irregular surface. Shot blasted alumina tends to become imbedded in and/or leave a residue on the surface of some stainless steels; therefore, subsequent steps of glass bead blasting and electropolishing are required.
While the surface finishing processes employed in the manufacture of stainless steel orthopedic implant devices have been generally successful in providing devices having clean surfaces, it is desired to develop a surface finishing process that maintains presently attained levels of surface cleanliness while further enhancing the fatigue properties of the device.